The present application relates to power MOS structures with inherent low forward voltage drop and fast switching diode characteristics, and more particularly to monolithically integrated gated power MOSFETs, in which a power MOS transistor is shunted with a similar power MOS transistor which has a lower threshold voltage.
Note that the points discussed below may reflect the hindsight gained from the disclosed inventions, and are not necessarily admitted to be prior art.
In MOSFET switches, and particularly at high switching speeds, reverse charge recovery Qrr contributes significantly to switching power losses. For example, in n-channel DC-DC converters, the transistor can at times operate in the third quadrant, i.e when its body diode is forward biased, which results in stored minority charge. This stored minority charge, in turn, causes some time delay in turning off the transistor.
Schottky barrier diodes (SBD) are often used as free-wheeling diodes in many converter topologies to address this problem. The Schottky barrier diode may be electrically connected in parallel with the body junction, since the Schottky barrier diode provides a lower forward voltage drop, and avoids minority carrier injection. The Schottky barrier diode will thus also typically have a lower stored charge in reverse recovery, which reduces switching losses.
Monolithically integrated MOSFET-SBD structures such as Trench MOSFET Barrier Structures (TMBS) have been proposed to lower Qrr in power MOSFETs. FIG. 7A shows a generic implementation of a monolithically integrated MOSFET and SBD structure. FIG. 7B shows one example of a monolithically integrated structure that includes one or more MOSFET sections adjacent to one or more SBD sections using Recessed Field Plate (RFP) trenches.
Trench MOSFET barrier structures such as shown in FIG. 7A suffer from relatively higher leakage current at reverse bias, and are also subject to additional process complexities to include a Schottky barrier in the MOS process flow.
New power MOSFET structures (for example in U.S. Pat. Nos. 7,843,004 and 8,076,719 and applications US 2010-0219462 A1 and US 2011-0254088 A1, which are hereby incorporated by reference) include Recessed Field Plate (RFP), Embedded Field Plate (EFP), Embedded Shielded Field Plate (ESFP) and quasi-vertical planar gate structures. Such MOSFET structures provide low specific on-resistance, gate-drain charge Qgd and lower gate charge Qg. However, to further reduce switching power losses, a reduction in reverse recovery charge Qrr is also needed.